Printing at higher speed in an ink jet printer makes it necessary to increase the speed of travel of a paper web in its printing direction and to decrease the interval between ink jets. Due to a limit for the frequency of voltage signal for ink jet, however, there is a limitation of decreasing the interval between ink jets. In JP 2,644,064 B, there is disclosed a printer in which a plural number of ink jet heads are arranged in a paper printing direction to print in a state that their ink jet heads complement one another, thereby to raise the printing speed while maintaining a printing density of, e, g., 600 dpi, constant in the paper traveling direction.
In the prior art mentioned above, two ink jet heads are arranged in two rows in the paper printing direction. With one ink jet head of them designed to deliver ink jets roughly to attain a printing density of 300 dpi at a maximum speed of travel of paper, the other ink jet head likewise designed to deliver ink jets at the 300 dpi printing density gives rises to increasing the printing density in the paper printing direction by doubling it, to 600 dpi. Then, dropping the printing speed (printing length per unit time) to one half while maintaining the ink jet interval at a minimum further raises the printing density in the paper printing direction to 1,200 dpi. However, the printing density in the direction (direction of paper width) orthogonal to the printing direction, namely in the direction of a nozzle row is fixed by the nozzle pitch; for example, a printing density of 600 dpi with a nozzle pitch of 0.042. Thus, while the printing density in the paper printing direction can be increased to 1,200 dpi, the printing density in the nozzle row direction remains at 600 dpi and cannot be raised. To effect printing at a high density of 1,200 dpi×1,200 dpi, therefore, there is a need to exchange with a separately prepared ink jet head having a nozzle pitch corresponding to the printing density of 1,200 dpi. However, it is difficult to maintain the accuracy of the ink jet head position after exchange thereof and time and work are necessary to adjust its accuracy.
With the aforementioned taken into account, it is an object of the present invention to provide an ink jet printer comprising a plural number of line heads arranged in a paper printing direction, each line head having a row of ink jet nozzles, in which for a mode of printing at a high definition in the paper printing direction due to a lowered printing speed, the printing density in a direction of the nozzle row can be rendered high in a simple operation and without the need to replace with such a head.